[circuitz]

This week I took delivery of a 3 axis TB6560 controller board - the kind that can be found all over eBay. On examining the board and I began to think about its circuit topology.

The ground fill on the underside of the board got my interest.
That ground fill goes all the way from the GND on the DB25 to the power connector GND at the opposite edge of the board >> WHAT!!

Therefore, on this popular board despite having "opto-isolators", there is NO ELECTRICAL ISOLATION BETWEEN THE PC AND THE STEPPER POWER SUPPLY.

There are many bad things that can occur because of this.

Could someone please confirm.

[Al_The_Man]

It is not necessarily all bad, it depends on if you subscribe to grounding power supply commons, the fact that the PC common is usually at earth ground potential anyway via the MB grounding, then if I were using that I would take a bonding conductor from the GND plane terminal to the central earth star point in the enclosure, but that is just me. :devious:
Al.

[circuitz]

Connecting power supply commons at one central point is good.

But if a bad (resistive) connection exists between this ebay TB6560 driver board and the 0V of the stepper PSU, the return current would rather flow along the printer cable, across the motherboard and into the PC power supply to where 0V is bonded to earth. Now the current is free to travel around the earth circuit of your shop to where the stepper power supply is plugged and eventually return to the 0V connection of the stepper PSU. In short a giant earth loop - assuming the PC is a desktop and not a laptop running on batteries.

In the meantime the voltage drop from motor currents surging along the printer cable interferes with signals into and out of the driver board.

OTOH if the frame of the CNC becomes 'live' then a very healthy current will seek to release all the digital smoke in every semiconductor device it chances to meet...PC included.

[Al_The_Man]

Connecting power supply commons at one central point is good.

But if a bad (resistive) connection exists between this ebay TB6560 driver board and the 0V of the stepper PSU, the return current would rather flow along the printer cable, across the motherboard and into the PC power supply to where 0V is bonded to earth.

This is why I mentioned a bonding conductor from the GND terminal, which evidentally is common to the P.Port common and hence the PC common which is grounded in the PC, this step should eliminate the stray currents across the MB, In fact this is how I have configured the systems I have installed so far.
Generally the PC ground is from a separate source, wall socket etc, and is usually not connected to the enclosure star point unless steps are made to bond it so.
Al.

[circuitz]

I can see how that would tame or redirect any stray currents. I just wish the opto-coupler in this particular product had been correctly applied to keep the PC electrically isolated from the machine controller and its power supply.

[Al_The_Man]

I actually bond the DC common of my servo supplies to the Earth Ground point.
And have never had a problem.
Also as I have pointed out before, if you have a BOB between the PC and say a Universal Router motor, which are notoriously noisy, electrically, as well as audibly, there is no isolation between it and the PC due to the grounded neutral.
Al.